In a chemical synapse, a nerve impulse can travel in only one direction. In contrast, in an electrical synapse, the impulse travels in both directions.

Also, across a chemical synapse, the impulse is transmitted with a 0.5-millisecond delay, while across an electrical synapse, the delay is almost non-existent.

SYNAPSES

The term “synapse” designates
the point where the axon of one neuron connects to a dendrite
of another. This word comes from the Greek syn (together) and haptein (join).
In the animal kingdom, neurons can be connected to each other in
two very different ways:

by an electrical synapse, in which the two cells touch and
are connected by tiny holes, which lets the nerve impulse pass
directly from one neuron to the other; or

by a chemical synapse, where the two cells do not touch and
the nerve impulse needs particular molecules to bridge the gap
between them.

Chemical synapses are slower than electrical
ones but are also far more flexible. This valuable flexibility is
the foundation of all learning.

On either side of the synapse, the axon
and dendrite have evolved specialized
molecules to perform their respective tasks.

NEUROTRANSMITTERS

Neurotransmitters are chemical molecules
that “ferry” nerve impulses across the synapse from
one neuron to the next.

Each type of neurotransmitter
has a molecular
form that lets it bind to the right site on the second
neuron to produce its particular effect.

The neurotransmitter thus acts somewhat
like a key. If it is the right shape for the next neuron
(shown here as a lock), it will produce an effect on that
neuron.

Neurotransmitters are divided into two
categories according to the effect that they have on the
second neuron once they are released into the synaptic
gap.

Neurotransmitters that help
this neuron to propagate the nerve impulse are classified
as excitatory neurotransmitters.

Neurotransmitters that reduce
the likelihood of this neuron’s propagating the
impulse are called inhibitory neurotransmitters.